Forward in situ combustion method for recovering viscous hydrocarbons



REFERENCE V5,: ($3 iweua Sept. 3, 1968 L. K. STRANGE FORWARD IN SI'IU COMBUSTION METHOD FOR RECOVERING VISCOUS HYDROCARBONS Filed April 1, 19s? 2 Sheets-Sheet 1 FIG. I

INVENT'QR LLOYD K. STRANGE ATTORNEY train-@3 3 P Sept. 3, 1968 L. K. STRANGE FORWARD IN SITU COMBUSTION METHOD FOR RECOVERING VI SCOUS HYDROCARBQNS Filed April 7, 1967 2 Sheets-Sheet 2 INVENTOR l +1.11 T L z w w L FIG. 3

LLOYD K. STRANGE Y ATTORNEY s 399 721 FORWARD IN srru coMisus'rIoN METHOD FOR RECOVERING viscous nvonocannoms A Lloyd K. Strange, Grand Prairie, Tom, assignor to Mobil Oil Corporation a corporation of New York Filed Apr. 7 1967, Ser. No. 629,275 4 Claims. (Cl. 166-2) ABSTRACT OF THE DISCLOSURE This specification describes:

A method for the recovery of viscous hydrocarbons from a subterranean formation employing forward in situ combustion. A fracture extends, between first and second spaced wells, in the formation. Fluid can flow only between the first well and the fracture. However, fluid can flow between the entire formation thickness and the second well. A forward combustion front is passed into the formation a certain distance from the second well. The

' area traversed by the front is then made water-wet. Thereafter, another forward combustion front is passed into the formation principally along thefracture from the first well. This front is moved toward the second well until it has substantially traversed the. formation between the spaced wells. Then, the formation between these wells is saturated with water introduced thereinto from the first well. During the preceding operations, displaced oil may be recovered from one, or both, of the wells. The latter combustion front is moved forward in the formation by a combustion-supporting gas. This gas is controlled in its flow for substantial oxygen utilization at the combustion front and the appearances of only minimal amounts of free oxygen at the second well, The free oxygen appearing at the second well is prevented from causing destructive combustion therein by employing a cooling fluid.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to thermal recovery methods for producing viscous hydrocarbons from a subterranean formation; and, more particularly, it relates to its natural state through a well by ordinary production methods. The oil sands along the Athabasca River in Alberta Province, Canada, which are commonly known as the Athabasca tar sands, are a prime example of formations containing hydrocarbons unrecoverable by conventional petroleum producing methods at the present time. Another example is the shallow sandstonelike formations found in California which contain such viscous crude oils that their recovery by conventional well production methods is unsatisfactory. The hydrocarbons in these oil sands have great viscosities at ambient formation temperatures but advantageously undergo a substantial reduction in viscosity when subjected to elevated temperatures. For this reason, thermal recovery procedures appear to bear great promise as a practical and an eco-' nomical method'for winning these hydrocarbons from oil sands.

.(2) Description of the prior art One thermal recovery method which can be used to great advantage in recovering viscous hydrocarbons from a. subterranean formation employs forward in situ com- 3,399,721 Patented Sept. 3, 1968 bustion. In this method, a combustiomsupporting gas is' passed through the formation between spaced wells and the innate carbonaceous material is heated to ignition temperatures. Under these conditions, a combustion front is created in the formation. This combustion front moves concurrently with the flow of the combustion-supporting gas. Air is generally used as the combustiomsupporting gas. Downhole heaters and burners may be used for igniting the carbonaceous material in the formation. The combustion front which results from these procedures can be termed a direct or forward combustion front because it moves in the direction in which the combustion-supporting gas fiows. The forward combustion front tends to displace hydrocarbons through a formation at a rate greater than they can flow into a production well. Under these circumstances, a bank of viscous hydrocarbons is created before the front which can effectively terminate the combustion front. Thus, there is a problem in producing the viscous hydrocarbon at suitable rates into a production well before an advancing combustion front. Additionally, viscous hydrocarbons are not readily moved through the'formation when both are relatively cold. For this reason, it would be advantageous to facilitate the flow of the cold viscous hydrocarbons through the formation and into the production well. Also, after the combustion front has traversed the formation and approaches the production well, the temperatures within the production well begin to increase. The temperatures may be sufficient that any free oxygen entering the production well will produce downhole combustion of the produced hydrocarbons. In this regard, it would be advantageous to prevent the occurrence of this .undesired combustion within the production well and the accompanying destructive temperatures.

The formation swept by the combustion front is a heat sink, void of fluids, into which viscous hydrocarbons from the surrounding area can flow. This is undesirable since these hydrocarbons may not be recovered by thermal recovery treatments of the adjacent formation. Additionally, the heat remaining in this formation is wasted.

As in any hydrocarbon recovery method, economics plays an important role. For example, air is commonly used as the combustion-supporting gas. Although the source of air is the atmosphere, moving the air into the formation at suitable conditions for advancing a combustion front becomes expensive. For this reason, controlled utilization of the free oxygen in the combustionsupporting gas is necessary to produce viscous hydrocarbons at the least expense.

7 SUMMARY OF THE INVENTION The present method provides for the recovery of viscous hydrocarbons from a subterranean formation by employing forward in situ combustion. A fracture in the lower portion of the formation fluidly connects spaced wells. A first well allows fluid flows only to the fracture. A second well is in fluid communication to the entire formation thickness. A forward combustion front is moved toward the first well a short distance into the formation from the second well. Then. the burned area swept by this front is saturated with water to make it water-wet. Thereafter, a forward combustion front is moved, by a concurrent flow of combustion-supporting gas, from the first well along the fracture into the formation. This front is moved until it resides adjacent the second well and the formation has been substantially traversed by it. The formation is then saturated with water from the first well. Displaced oil is recovered from the second well; I

The combustion-supporting gas is controlled in its flow through the formation so that oxygen utilization at the combustion front is substantial, and no more than minimum amounts of free oxygen appear at the second well. For this purpose, adjustments can be made in the rate of llow, pressure, and composition of the combustionsupporting gas. Additionally, a cooling fluid may be introduced into the second well to prevent any free oxygen from producing destructive effects from combustion of produced hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 1 through 4 illustrate a vertical section taken through the earth of a subterranean formation provided with suitable structures for carrying out a forward in situ combustion method for recovering viscous hydrocarbons according to the present invention. The

figures show serially the successive several steps, and. their results, during the practice of this method in the formation.

DESCRIPTION OF SPECIFIC EMBODIMENTS in the drawings and the following description, like structures will be designated with like nomenclature and numeral.

Referring to. FIGURE 1 of the drawings, the method of this invention will be described in reference to a subterranean formation, which contains viscous hydrocarbons, and carries suitable apparatus for practicing the method. A subterranean formation 11, which may be'a heavy oil formation or a tar sand, resides below an overburden 12 and the earths surface 13. The overburden 12 may be a thin layer of soil mantle of glacial drift or other strata. The formation 11 rests upon a bedding, which may be limestone. The formation 11 may be variable in thickness, for example, from several feet to several hundred feet in thickness. Similarly, the hydrocarbons may have a substantial range in their amounts present, and their physical properties,

in the formation 11.

Spaced-apart wells 16 and 17 are extended from the earths surface 13 downwardly into the formation 11.

Preferably, the wells 16 and 17 extend substantially the entire thickness of the formation 11. The well 16 is arranged to provide fluid communication between the earth's surface 13 .and only a limited lower portion of the formation 11. For this purpose, any means of construction in the well 16 may be employed. Preferably,

the well 16 is provided with a casing 18 extending the full thickness of the formation 11. The casing 18 is secured to the surrounding surfaces of the formation 11 about the well 16 by a cement sheath 19. With the sheath 19 in place, a circumferential notch 21, in the horizontal, is provided through the casing 18 andsheath l9 and extends a short distance radially into the formation 11. Thenotch 21 resides at the lower portion of the formation 11 where fluid communication from the well 16 is desired. I

Thereafter, a' horizontal fracture 22 is extended through the formation 11 from the notch 21 at the well 16 towards the well 17. The fracture 22 may be ,provided by. any suitable fracturing mode. For example, the well 16 is provided at its upper extremity with a wellhead 23. The wellhead 23 carries tubings 24 and 7 or' for other reasons.

The well 17 is arranged for fluid communication over substantially the entire vertical extent of the surround- 11. A plurality of openings, such as perforations 28, in casing 27 provides for fluid communication between the well 17 and the surrounding formation 11. The casing 27 carries a wellhead 29 at its upper extremity. A tubing 31 extends downwardly through the wellhead 29 into the casing 27 and terminates adjacent the lower extremity of the formation 11; Additionally, a tubing 32 is provided the well 17 for fluid communication to the annulus between the casing 27 and the tubing 31.

The wells 16 and 17 may be provided with the usual seals against fluid flow where they pass through the overburden 12. Also, the wells 16 and 17 are sealed against fluid fiows at their lower extermities adjacent the bedding 14. It will 'be apparent from the structures described for use with the wells 16 and 17 that suitable means are provided for passing fluids between the earths surface 13 and the formation 11. Additionally, the usual apparatus for moving the various fluids can be employed for carrying out forward in-situ combustion with the wells 16 and 17. This apparatus is considered conventional, and therefore, it is not shown or described with the present embodiment.

Referring now to FIGURE 2, a combustion-supporting gas, supplied to the tubing 31 or 32 from any suitable source,is passed from the well 17, via perforations 28, to flow into the formation 11 and towards the well 16. At this time; fluids are produced from the well 16' via tubings 24 or 26. The combustion-supporting gas may have any composition which can sustain combustion of the carbonaceous materials which reside in the formation 11. Usually, the combustion-supporting gas will be air, or mixtures ofair with various other gases to include additional oxygen, fuel, or inert diluents. Whatever the source or composition of the combustionsupporting gas, it must contain free oxygen which supports combustion of the canbonaceous material present within the formation 11.

The carbonaceous material in the formation 11 about the well 17 is now ignited to create a combustion front. In many instances, the passage of the combustionsupporting gas into the formation 11 causes an increase in temperatures, by auto-oxidation, sufiicient to ignite carbonaceous materials in the formation 11 about the well 17. However, other means for igniting these materials may be employed. For example, an electrical heater placed within the well 17 adjacent the perforations 28 can be activated to heat these materials to ignition temperatures. For this purpose, it is usually desirable to remove the tubing 31 from the well 17. With the heater activated, the passage of the combustion-supporting gas will result in a combustion front being produced in the formation 11 about the well 17.

The resulting combustion front is moved, by the flow of the combustion-supporting gas, towards the well 16. This front leaves behind in the formation 11 a burned area 33 about'the well 17. The burned area 33 is substantially depleted of viscous hydrocarbons and other carbonaceous materials.- It is also heated to elevated temperatures at which the viscous hydrocarbons are readily thinned to a degree where they flow readily through the formation 11. The burned area 33, usually should extend radially at least a distance of about 5 feet into parts of the formation 11 from'the well 17. It has been indicated ing formation 11. For this purpose, the well 17 has a casing 27 extending downwardly through the formation that with a burned area 33 of such dimension, no liquid blocks normally occur in the flow of the viscous hydrocarbons in the formation 11 displaced before a forward combustion front moving towards the well 17. After the burned area 33 is obtained, the flow of the combustionsupporting gas from the well 17 through the formation 11 is terminated. I

The fracture 22 is of particular advantage in the pro duction of the burned area 33 for, carrying the viscous hydrocarbonsdisplaced before the forward combustion front from. the burned area 33 adjacent the well 17 toward, and into, the well 16. The hydrocarbons produced into the well 16 can be recovered by means of the tubing 24. ThusQby creating the burned area 33, no liquid blocking will occur adjacent the well 17 to extinguish the forward combustion front.

Water is introduced from the well 17 in a sufficient amount to saturate the voids and interstices in the burned area 33.-The water is injected from the well 17 under such conditions of pressure and temperature that it is in a liquid phase in the burned area 33. Preferably, the water traversing at least parts of the burned area 33 has, correctcd to atmospheric pressure, a temperature of not over about 200 F. For best results, the water should be as hot as possible but. yet remain in the liquid phase. By these steps, the formation 11 adjacent to, and surrounding, the well 17 is maintained water-wet by preventing oilwctu'ng on displacement of oil into the burned area 33. Viscous hydrocarbons more easily flow through the formation 11 when water-wet than in its original oil-wet state. After these steps, the well 17 is arranged for removing fluids from the formation 11 through the tubing 31. I

Referring now'to FIGURE 3, combustion-supporting gas is introduced through either tubing 24 or 26 into the well 16. This gas then flows from the notch 21 into the fracture 22 and through the formation 11 into the well 17. At this time, fluids are removed from the well 17 via the tubing 31. The formation 11 adjacent the fracture 22 and the notch 21 is heated to suitable temperatures so as to ignite the carbonaceous materials which reside therein. Any suitable means may be employed within the well 16 for this purpose. For example, the flow of combustion-supporting gas may heat, by auto-oxidation, the formation 11 next to the notch 21 to ignition temperatures. However, other means may be used to obtain ignition temperatures, such as the heater earlier described for use in the well 17. As a result of these steps, a forward combustion front is produced. This from moves toward the well 17 through the formation 11 principally along the fracture 22. As this combustion front traverses the formation 11, viscous hydrocarbons are displaced from the formation 11 into the well 17. These hydrocarbons are recovered from the well 17 through the tubing 31.

The combustion-supporting gas is controlled in its flow from the well 16 through the formation 11 to obtain a specific result. More particularly, the combustion-supporting gas is controlled to maintain the free oxygen appearing at-the well 17'to an amount that combustion therein of produced hydrocarbons is minimized, and also that substantiallyall of the free oxygen in the combustionsupporting gas is utilized in moving the combustion front through the formation 11. Any means can be employed for monitoring the free oxygen appearing in the well 17.

For example, an oxygen analyzer may be installed on the tubing 31 to monitor free oxygen in the gases removed from the well 17. Additionally, the temperature within the well 17 indicates when amount of free oxygen present in the produced fluids causesdownhoie combustion. Obvi ously, the temperature within the well 17 increases rapidly as combustion of the produced hydrocarbons occurs within the well 17. At the same time, free oxygen decreases in the produced gases. Combustion of produced hydrocarbons inthe well 17 is very obviously undesirable in that unprotected downhole apparatus in the well 17 is heated to destructive temperatures. Under these condiliOnS, thermal damage occurs. to metallic structures in the well 17. The costly effects from corrosion and erosion are greatly increased.

The combustion-supporting gas can be controlled to provide the above results by adjusting one or more conditions whichaffect combustion. The rate and pressure of gas flow from the well- 16 into the fracture 22 can be adjusted-Obviously, either, or both, of these flow conditions controls the amount of free oxygen which appears at the well 17-. if desired, a hydrocarbon gas can be added to the combustion-supporting gas to provide additional fuel for the oxygen present therein to consume at the combustion front. Thusly, a greater utilization of the oxygen at the combustion front is produced. Alternatively, an inert gas may be added to the combustion-supporting gas to dilute it to a reduced oxygen content. which content likewise controls the amount of free oxygen appearing at the well 17.

Additionally, it is desirable that the temperature rn the well 17 be reduced as the combustion front traverses the formation 11 and approaches it. For this purpose, a cooling fluid, such as water, is circulated in the well 17. For example, a cooling fluid is introduced through the tubing 32 to flow downwardly in the annulus between the casing 27 and the tubing 31. Thereafter, the cooling fluid is removed to the earth's surface 13 through the tubing 31 with the hydrocarbons which enter the well 17. Preferably, the introduction of cooling fluid into the well 17 is practiced in conjunction with the above steps of controlling the combustion-supporting gas in its flow to main tain the free oxygen in the well 17 at a minimum while reducing downhole temperatures in the well -17.

lt'will be apparent that the fracture 22 facilitates the flow of the cold viscous hydrocarbons displaced before the last-mentioned combustion front in its movement through the formation 11 toward the well 17. These hydrocarbons readily flow through the-fracture 22 into the lower extremity of the well 17 from which they are readily recovered through the tubing 31. Additionally, the waterwet burned area 33 also provides a ready flow path in the formation 11 for the viscous hydrocarbons flowing outside of the fracture 22. For example, the hydrocarbons displaced in the upper portions of the formation 11 flow into the well 17 through the burned area .33 with great facility because of its water-wet condition, andalso because such flows are over the substantial vertical extent of the formation 11. Additionally, as the hydrocarbons displaced by the combustion front become heated, they tend to remove some of the water from the burned area 33. This water flows into the well 17 with the produced hydrocarbons and assists in cooling it. As a result, the combustion front more readily displaces the viscous hydrocarbons from the formation 11 into the well 17. The danger of liquid blockage about the well 17 is avoided. Also, the well 17 is protected from destructive temperatures and corrosion problems.

As the combustion front traverses the formation 11 in its movement toward the well 17, a front-swept area 36 of the formation 11 becomes heated to elevated temperatures and stripped of most hydrocarbons. The combustion front is terminated after moving substantially across the formatiori 11 to adjacent the-well 17. For this purpose, the how of combustion-supporting gas is terminated to extinguish the front before the well 17 is destroyed.

With reference to FIGURE 4, after the combustion front has been terminated adjacent the well 17, water is injected into the formation 11 from the well 16 through the notch 21. The water is injected at suitable conditions of pressure and temperature to saturate the formation 11 which has been traversed by the combustion front. This front-traversed formation 11 has been heated to temperatures of up to about 1200" F., although the resulting average temperature will be somewhat less. Thus, the water flowing into the formation 11 under these conditions is heated. In being so heated, the water distributes this residual heat from the combustion front throughout a large area 37 of the formation 11 which resides between the wells 16 and 17.' Additionally, the heated water displaces additional quantities of hydrocarbons into the well 17 from outside the front-swept area 36. Thus, the saturation with water of the formation 11 provides for scavenging the heat which remains after the combustion front has passed through the formation 11. Additionally, the water saturation provides'for an immiscible displacement by hot water, of hydrocarbons which remain in the formation 11 between the wells 16 and 17. After the area 37 is saturated with water, the injection of the water is terminated. The water-saturated area 37 is of particular combustion comprising the steps of:

advantage when thermal recovery operations are per formed nearby since any displaced fluids cannot flow into that part of the formation 11 traversed by the combustion front. Where the area 37 would remain a burned-out void, hydrocarbons could readily fiow into it and there remain unrecovered by thermal recovery operations carried on in adjacent areas of the formation 11.. However, the watersat'urated conditions of the area 37 prevent this undesired result.

The hydrocarbons displaced through the formation 11 by practice of the various described steps are removcred through one of the wells 16 and 17. It will be apparent that the displaced hydrocarbons can be recovered on a continuous basis while carrying out the steps of this method, or on an intermittent basis conjunctively with, or separate from, the practice of any of these steps.

From the foregoing, it will be apparent that a method has been disclosed which overcomes many of the aforedescribed problems present in the production of viscous hydrocarbons from a subterranean formation. Various modifications of the disclosed method may be made by those-skilled in the art without departing from the spirit of this invention. Similarly, the disclosed steps of this method may be employed in combination with, and in subcombinations of, other steps for producing viscous hydrocarbons. For this and other reasons, the present description is intended to be illustrative of this invention.

What is claimed is: l. A method for recovering viscous hydrocarbons from an earth formation to be subjected to forward in situ (a) providing fluid communication, at spaced-apart locations, between the earths surface and a limited lower portion of the formation at. a first well means and over substantially the vertical extent of the formation at-a second well means;

- (b) providing a horizontal fracture from the limited lower portion of the formation at the first well means through the formation to the second well means;

(c) passing a combustion-supporting gas through the formation from the second to the first of the well means, and igniting carbonaceous material in the formation about the second well means to' provide a combustion front moving toward the first well means;

(d) moving the combustion front from the first to the second of the well means to provide a burned area in the formation about the second well means, said area beingsubstantially depleted of viscous hydrocarbons and at least in part of the formation extending radially from the second well means at least a distance of about five feet in which liquid blocks normally would not occur in viscous hydrocarbons displaced before an advancing forward combustion front, and on forming the burned area, terminating the passing of the combustion-supporting gas through the formation;

,(e) injecting water, in a liquid phase, from the second wellmeans into the formation in a sufficient amount to saturate the burned area thereby providing a water-wet condition in the formation about the second well means, the water traversing at least a part 8 of the burned area having a temperature of about 200 F (f) passing a combustion-supporting gas into the. fracture and through the formation from the first to the second well means, and igniting carbonaceous material in the formation adjacent the fracture in the lower portion of the formation and adjacent the first well means to provide a combustion front which moves toward the second well means for displacing hydrocarbons into the second well means;

('g) controlling the combustion-supporting gas in its flow through the formation to maintain the free oxygen, appearing at the second well means, to an amount that combustion therein of produced hydro carbons is minimized and substantially all of the free 7 oxygen in the combustion-supporting gas is utilized in moving the combustion front from the first to the second well means;

(h) terminating injection of the combustion-supporting gas when the combustion front has moved substantially across the formation to adjacent the second well means and before destructive temperatures are generated therein;

(i) injecting water into the formation from the first well rneans to saturate the'formation traversed by the last-mentioned combustion front; and

(j) recovering the hydrocarbons displaced by the preceding steps (a)(i) from the formation into said well means.

2. The method of claim 1 wherein in step (g) the rate and pressure are adjusted in the flow of the combustionsupporting gas being introduced into the formation to control the amount of oxygen present at the second well means.

3. The method of claim 1 wherein in step (g) a hydrocarbon gas is added to the combustion-supporting gas being introduced into the formation to control the amount of free oxygen present at the second well means.

4. The method of claim 1 wherein the combustionsupporting gas is controlled as to the amount of free oxygen appearing at the second well means to reduce the .combustion occurring therein with produced hydrocan hens, and a cooling fluid is circulated through said second well means to.reduce do'wnhole temperatures within the second well means.

' References Cited UNlT ED STATES PATENTS 7/1966 Gates u 166-11 STEPHEN LINOVOSAD, Primal- Examiner. 

